1. Field of the Invention
This invention generally relates to a technology of surface inspection of a magnetic disk to be used as storage medium of computers after the completion of a series of predetermined surface treatment steps in the process of manufacturing the disk.
2. Related Art Statement
Magnetic disks to be used as storage medium of computers are typically made of aluminum alloy or glass and have a central through hole therein and a magnetic film formed on the surface thereof.
For preparing a disk from aluminum alloy, it is typically punched out of a plate of aluminum alloy by means of a press to show predetermined inner and outer diameters and then subjected to a series of surface treatment steps including grinding, lapping and polishing, after which the surface of the disk is inspected to see if it meets given precision requirements.
In order to continuously and efficiently inspect disks in known arrangements for manufacturing disks on a mass production system, disks are rotatably held on an indexing table, namely an indexing rotary table adapted to revolve by a predetermined indexing angle and move the discs in turn to a loading station where one or more than one disks are loaded on the indexing table, to an inspection station where the disks are inspected for the surface, and then to an unloading station where the disks which have been inspected are unloaded from the indexing table and delivered to the outside. The indexing table is provided with chucks arranged angularly at a predetermined regular pitch for holding disks.
Since a magnetic disk is prepared by laying a magnetic film on the opposite surfaces of a blank disk, it is normally turned upside down on a same indexing table so that the two surfaces may be inspected on the same table. Alternatively, a pair of indexing tables may be used so that each magnetic disk is inspected on one of the tables for one side thereof and then moved onto the other table, where it is inspected for the other side.
With either system, spot light such as laser spot light is made to strike the disk which has been moved to the inspection station by the indexing table and the reflected light is used to inspect the disk for the surface and find out if the surface is damaged or not and if foreign matters such as dirt is adhering to the surface or not. More specifically, the disk is driven to rotate so that it maybe scanned radially by spot light to inspect the entire surface.
In an attempt to inspect the disk efficiently within a short time, the disk held by a chuck is caused to revolve typically at a rate of about 6,000 rpm by the chuck during the inspection. With such a high rate of revolution, the surface of each disk can be inspected in about seven seconds.
However, with the above described arrangement for examining a disk for the surface, the chuck is made to pass through the central through hole of the disk and rigidly hold the disk. Then, care should be taken not to tighten the chuck too hard in order to prevent the peripheral surface of the central through hole from being damaged by the chuck. Therefore, a problem of relative displacement between the disk and the chuck can arise due to the relatively loose chuck when the disk on the inspection station that is initially not revolving is driven to get to a predetermined high rate of revolution within a very short period of time. This problem is currently avoided by taking a predetermined time for an initially not revolving disk to get to the predetermined rate of revolution for inspection and, conversely, for a disk revolving at a predetermined high rate of revolution to get to a complete halt after the end of inspection.
Thus, conventionally, after getting to the inspection station on a indexing table, the disk to be inspected on the indexing table is driven to rotate until it gets to a predetermined rate of revolution, when an operation of inspecting the disk starts. With such an arrangement, the total time requirement will be the sum of the time required for the proper inspection of the disk and the time necessary for the disk to get to a predetermined rate of revolution at the inspection station.